A solid polymer electrolyte fuel cell (Polymer Electrolyte Fuel Cell: hereinafter called “PEFC” as needed) has been developed as a fuel cell vehicle, a home cogeneration system, and the like.
As a catalyst used for the gas diffusion electrode of PEFC, a noble metal catalyst composed of a noble metal of platinum group elements such as platinum (Pt).
For example, as a typical conventional catalyst, there has been known “Pt on carbon catalyst” (hereinafter called “Pt/C catalyst” as needed) (for example, Pt/C catalyst having a Pt support rate of 50 wt %, Trade Name: “NE-F50” available from N.E.CHEMCAT).
In the preparation costs of PEFC, a proportion of the noble metal catalyst such as Pt is large, and it is the problem to lower the PEFC cost and to spread PEFC.
To solve the problem, developments of technique for lowering the noble metal in the catalysts, or technique for de-noble metalizing have been progressed.
Among these developments, in order to reduce the amount of platinum to be used, a catalyst particle having a care-shell structure formed by a core part made of non-platinum element and a shell part made of Pt (hereinafter called “core-shell catalyst particle” as needed) has been studied, and there are many reports.
For example, in Patent Document 1, there is disclosed a particle composite material (core-shell catalyst particle) having a structure where palladium (Pd) or a Pd alloy (corresponding to the core part) is covered with an atomic thin layer of Pt atom (corresponding to shell part). Further in Example of this Patent Document 1, a core-shell catalyst particle where the core part is a Pd particle and the shell part is a layer made of Pt is described.
In addition, there has been studied a structure where a metal element other than the Pt group is contained as the structural element of the core part. To the contrary, there has been also proposed a structure where a metal element other than the Pt group is contained as the structural element of the shell part.
For example, as a structure containing tungsten (W) as the structural element of a core part, a structure having a core part made of simple W, a W alloy, a W oxide has been proposed (for example, Patent Documents 2 to 9).
In addition, as a structure containing W as the structural element of a shell part, a structure having a shell part made of simple W, a W alloy, a W oxide has been proposed (for example, Patent Document 10).
In more detail, Patent Documents 2 to 5 disclose structures having a core part containing a W oxide.
In Patent Document 2, there is disclosed a synthesis example of a catalyst having a structure that particles where a core part is WO2 and a shell part is an alloy of a reduced product of WO2 (WO2y, 0<y≦2) and Pd are supported on a carbon support (Patent Document 2, Example 8).
In Patent Document 3, there is disclosed a platinum-metal oxide composite particle where a core part is made of a W oxide (sodium tungsten oxide, etc.) and a shell part is made of Pt, etc.
In Patent Document 4, there is proposed a catalyst particle having a structure where a base particle (care part) is made of a metal oxide particle made of two or more solid solutions selected from simple W or a metal element group containing W and a metal coating layer (shell part) is made of a solid solution selected from simple Pt or a metal element group containing Pt.
In Patent Document 5, there is proposed a catalyst particle having a structure where a base particle (core part) is made of a W oxide, and at least of a part of the base particle surface is coated with one or more metals (shell part) such as Pt.
Further, Patent Documents 6 to 9 disclose structures having a core part containing simple W, or a W alloy (W solid solution).
In Patent Document 6, there is disclosed a catalyst particle where an inner core (core part) is made of simple W, an alloy of W and a metal selected from other metal group, or a mixture thereof, and an outer core shell part is made of Pt or a Pt alloy.
In Patent Document 7, there is disclosed a Pt-containing catalyst having a structure that a metal particle where a core particle (core part) is made of a metal atom other than Pt or an alloy of a metal atom other than Pt, and the surface of the core particle is coated with a shell layer (shell part) made of Pt is supported on an electrically conductive support. In this document, W is disclosed as the structural material of both of the core part and the shell part (Patent Document 7, Paragraph 0020, Paragraph 0021).
In Patent Document 8, there is disclosed a core-shell type fine particle which has a core particle (core part) having a face-centered cubic crystal structure made from a material of simple W or a W alloy, and a shell layer (shell part) having a face-centered cubic crystal structure made from a material of a metal such as Pt.
In Patent Document 9, there is disclosed a core-shell type fine particle which has a core particle (core part) made from a material of simple W or a W alloy, and a shell layer (shell part) made from a material of a metal such as Pt.
Further, though it is not clear as to whether or not the proposed particles correspond to a catalyst particle having a core-shell structure, there are proposed catalysts that Pt or a Pt aloy is supported on a W carbide particle as an electrode catalyst for a fuel cell (Patent Documents 11 to 12, Non-Patent Document 1).
In Patent Document 11, there is disclosed a catalyst where, in order to modify a surface of an electrically conductive carbon, a W carbide particle (particle of a mixture of WC and W2C, or particle made of WC) is produced on the electrically conductive carbon, and then Pt particles are supported on this particle.
In Patent Document 12, there is disclosed a catalyst where Pt particles are supported on a particle mainly made of WC. However a structure where the catalyst particle is supported on an electrically conductive carbon is not studied.
In Non-Patent Document 1, there is disclosed a catalyst where Pt particles are supported on a particle mainly made of W2C. However a structure where the catalyst particle is supported on an electrically conductive carbon is not studied.
Furthermore, in a catalyst particle having a core-shell structure formed of a core part made of a non-Pt element and a shell part made of Pt, there is proposed a structure where a catalyst activity is intended to be improved together with reduction of a Pt amount (for example, Patent Document 13).
For example, Patent Document 13 proposes an electrode catalyst fine particle for a fuel cell having a child core-shell structure which comprises a center particle (core part) containing a Pd alloy, a outermost layer (shell layer) containing Pt, and an intermediate layer consisting of simple Pd provided between the center particle and the outermost layer.
Incidentally, the present applicant submits, as publications where the above-mentioned publicly-known inventions are described, the following publications: